The present invention relates to methods and systems used to drive multiphase brushless DC motors.
The magnetic field used to turn a permanent magnet rotor is generated using three (or more) interconnected phase windings in the stator of the motor. In a bipolar mode the current is driven through only two phase windings at a time and the third undriven phase winding is used to monitor the back EMF voltage. By monitoring the back EMF voltage, the position of the rotor can be determined. This is achieved by detecting the zero crossing point of the monitored back EMF waveform within the phase winding, the zero crossing point occurring when the rotor is in a defined position. If the position of the rotor is known, the driving of the phase windings may be synchronised with the rotor position for maximum power. An example of such a method is disclosed in U.S. Pat. No. 6,034,493 (Boyd et al).
In a tri-polar (or multi-polar) mode of operation all three phase windings are driven. Typically the three phases carry currents having sinusoidal or trapezoidal waveforms with relative phases of 120 degrees. The advantage of such an arrangement is that the torque ripple caused by driving only two coils at any one time is then minimized.
Disadvantageously, in multi-polar mode since the windings are each being driven all the time, the back EMF cannot be sensed in the conventional manner. Accordingly, the back EMF in a coil is sensed during a period when the driving current to said coil is interrupted, typically in response to an external signal. A comparator or similar can then be used to determine the occurrence of zero crossing during this undriven period. Various examples of such methods are described in U.S. Pat. No. 7,141,949 (Harwood), EP1478086 (Matsushita Electric), EP0892489 (ST Microelectronics), US2004/263104 (Iwanaga et al) and U.S. Pat. No. 7,235,939 (Viti et al).
To reliably detect the zero crossing point, the zero crossing point must occur during the undriven period. Accordingly, the duration of the undriven period must be long enough to take into account likely delays or advances of the zero crossing point. Delay or advance of the zero crossing point typically occurs due to dynamic motor speed changes which might occur during a fast start-up phase or in response to load changes.
There is therefore a conflict between the opposing desires of reducing the duration of the undriven periods (to increase torque and reduce noise) and increasing the duration of the undriven periods of time (to reliably detect zero crossings and hence accurately estimate rotor position for control purposes).
An additional problem is that the back EMF is proportional to motor rotational speed, being large at high rotation speed and zero at standstill. Therefore the back EMF zero crossing can only be detected on a running motor. Additionally, at low motor speeds, the back EMF is small and thus system noise can swamp the back EMF signal. Accordingly, reliable rotor position estimation becomes more difficult as motor speed reduces. To avoid such problems many motors are adapted to operate only above a preset minimum motor speed. In some applications however a low motor speed is desirable, for instance to reduce energy consumption.
It is therefore desirable to provide alternative methods of determining the zero crossing point of a phase winding in a BLDC motor which at least partially overcomes or alleviates the above problems.